1. Field of the Invention
This invention concerns electrical discharge machining processes and improved dielectric fluids for use therein.
Electrical discharge machining (EDM) is a method for drilling, cutting or shaping electrically conductive stock by means of the controlled removal of material from the surface through melting or vaporization by high-frequency electrical sparks. The spark discharge is produced by controlled pulsing of direct current between the work piece and the tool or electrode. The end of the electrode and the work piece are separated by a narrow spark gap and are flooded by a dielectric fluid. The dielectric fluid in a gap is partially ionized under the pulsed application of a relatively high voltage, thus enabling a spark discharge to pass between the tool and the work piece. Each spark produces enough heat to melt or vaporize a small quantity of the work piece, leaving a tiny pit or crater in the surface of the work.
EDM is often used when the work material is of high hardness, high tensile strength or poor machinability and the product to be formed is of complex or irregular shape or fragile structure.
The dielectric fluid serves as a spark conductor and coolant, and as a flushing medium for removal of the small particles of material separated from the work piece. In practice, the dielectric fluid is recycled, being collected from the apparatus, filtered and then returned to the apparatus. Dielectric fluids have generally been selected for their high dielectric strength, i.e., low conductivity. Satisfactory fluids should have controlled dielectric properties so as to provide a sizable charge flowing from the electrode to the work piece, should be of a light color so that the work piece can be observed through the oil, should be able to carry detritus to the filter for filtration, and should be oxidatively and thermally stable for long periods of time.
Electrical discharge machining processes and EDM fluids are described in the "Metals Handbook", Vol. 3 (8th Ed., American Society for Metals, Metals Park, Ohio, 1967). EDM is also the subject of an article by Bonales in "Products Engineering", p. 53 (Sept. 27, 1965). EDM is further described in Chapter 31 of the text entitled "Manufacturing Processes and Materials for Engineers" (2d Ed., Prentiss-Hall Inc., 1969). All these are incorporated herein by reference. The voltages used are relatively high, although they are often below 100 V, while being sufficient to generate the pulsed sparks, as described more fully in the foregoing.
EDM and fluids therefor are also disclosed in U.S. Pat. Nos. 3,639,275, 3,648,013 and 3,679,857.
It is found that in some instances finely divided material accumulates in the dielectric fluid, thereby causing darkening of the dielectric fluid, which darkening is disadvantageous in EDM operations. The dispersed, finely divided particles accumulating in the fluid appear to originate from two sources, namely, solidified metal droplets melted off the work piece and electrode wear fragments created by the electric arc. The accumulation of dispersed particles appears to be greater with the newer EDM electrodes, particularly silver tungsten and copper-tungsten electrodes. The accumulation of dispersed particles is also aggravated in the smaller EDM machines, whose filtration systems are incapable of removing particles smaller than about 5 microns in size; since particles smaller than 1 micron are often formed in EDM machining. In the absence of adequate filtration, these dispersed particles circulate with the fluid and darken it as their concentration builds up.